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Sexual transmission of the Zika virus might not be as unusual as previously thought.

The Centers for Disease Control and state public health departments are investigating 14 additional reports of possible sexual transmission of the virus, including several involving pregnant women, according to a "health advisory" the CDC issued Tuesday afternoon. A health advisory is not as urgent as a "health alert" and "may not require immediate action," according to the CDC.

Sexual transmission is not surprising. Zika virus has been found in a variety of body fluids, including blood, urine, saliva and semen, and, according to the CDC, it remains in semen longer than in blood. (The Food and Drug Administration a week ago issued a new guideline recommending that people who have traveled to areas having a Zika outbreak, might have been exposed to the virus or have had a confirmed infection defer donating blood for four weeks. There had been no reports of Zika virus entering the U.S. blood supply, the FDA said last week.)

The CDC published guidelines Feb. 12 for prevention of sexual transmission of Zika virus. Those guidelines were based on reports of three cases, although the new health advisory notes that the CDC received a report of another case of Zika virus detected in semen after it published the guidelines. Because of the risk of microcephaly, a birth defect, men who live in or have traveled to an area of active Zika virus transmission should abstain from sexual activity or consistently and correctly use condoms if their partner is pregnant, according to the CDC. The same recommendation holds true for such men if their partners aren't pregnant but they're concerned about sexual transmission of the virus, the guidelines say, noting that illness caused by the Zika virus is usually mild. How long the Zika virus can remain in semen isn't known, according to the CDC.

The first of the three cases cited in the CDC guidelines dates back to 2011, when Colorado State University scientists published a report in Emerging Infectious Diseases, a CDC journal, about a Colorado man who was infected with the Zika virus during a trip to southeastern Senegal in 2008. After he returned home, he experienced symptoms of Zika infection as well as inflammation of the prostate, or prostatitis. Four days later, he saw blood in his semen; on the same day, his wife developed Zika infection symptoms.

Blood tests confirmed that both of them were infected. She had not traveled out of the United States in the previous year, but she and her husband had sexual intercourse a day after he returned home, suggesting the virus had been transmitted via his semen, although it was never tested.

But a year ago, scientists reported finding the Zika virus in a man's semen. The man became infected during what had previously been the largest known Zika outbreak, in which an estimated 28,000 people--or 11% of the population--in French Polynesia sought medical care for infection symptoms during 2013-2014.

The man reported having two bouts, eight weeks apart, of symptoms related to infection by the Zika virus. A couple of weeks after his second round of symptoms, he sought medical treatment because he had noticed blood in his semen. Although the man said he had not had any recent contact with people who had symptoms of Zika infection, tests found the virus in his semen and urine but not in his blood. That suggests that urine samples might be useful for late diagnosis of Zika infection in people who've already cleared the virus from their blood, the scientists wrote. The man had no sexual contacts.